1. Field of the Invention
The present invention relates to an image forming apparatus which comprises an endless transfer member capable of bearing thereon either a plurality of images of a specific size or a plurality of sheets of a specific size on which respective images are supported.
2. Description of the Prior Art
In most image forming apparatus such as copiers or printers, a toner image formed on a photoconductor or photoreceptor is transferred onto a sheet. Also, in multi-color image forming apparatus, a plurality of images having different colors, respectively, are successively transferred so as to superimpose one above the other on the same region of one sheet.
To superimpose a plurality of toner images on the sheet, U.S. Pat. No. 5,138,398 discloses an endless transfer drum capable of supporting a sheet onto which the toner images are transferred. In an image forming apparatus comprising such transfer drum, toner images formed on the photoconductor are transferred onto the sheet supported on the transfer drum. By introducing the sheet into a transfer region where the photoconductor faces the transfer drum repeatedly, the toner images are transferred onto the sheet so as to superimpose one above the other.
Further, to superimpose two toner images on the same sheet for forming a composite image, Japanese Patent Laid-open Publication No. Hei 2-247,663 discloses an image forming apparatus employing an endless transfer belt capable of supporting a sheet thereon.
Furthermore, U.S. Pat. No. 5,117,261 discloses another endless transfer belt on which a plurality of toner images of different colors are directly transferred and superimposed so as to form a multi-color image, the multi-color image thus formed on the transfer belt is then transferred onto the sheet in a lump.
The transfer member, i.e., the endless transfer drum or endless transfer belt described above has a circumferential length corresponding to the maximum possible size of sheets which can be printed by the image forming apparatus. Therefore, although in making an image on the maximum-size sheet the circumference of the transfer member is almost occupied by the sheet, making an image on a sheet of a smaller size allows the sheet to occupy only a part of the circumference, resulting, in an increase of waste time during which the transfer member does not contribute to the transfer.
U.S. Pat. No. 4,712,906 discloses an image forming apparatus wherein the endless transfer drum is so designed as to support one legal-size sheet or two letter-size sheets. In this image forming apparatus, even in the image forming using sheets of a letter size, the circumference of the transfer drum is almost covered by the sheets so that an efficient image forming process may be accomplished. However, this prior art does not refer to a controlling sequence for making a plurality sets of copies of a plurality of documents.